Optical properties of aqueous colloidal dispersions of 2D electrolytes, if their aspect ratios are extra-large, can be determined by their orientation preferences. Recently, we reported that a colloidal dispersion of diamagnetic titanate(IV) nanosheets (Ti(IV)NSs), when placed in a magnetic field, is highly anisotropic because (TiNS)-N-IV anomalously orients its 2D plane orthogonal to the magnetic flux lines due to its large anisotropic magnetic susceptibility. Herein, we report a serendipitous finding that Ti(IV)NSs can be in situ photochemically reduced into a paramagnetic species (Ti(IV/III)Nss) so that their preference of magnetic orientation changes from orthogonal to parallel. This transition distinctly alters the structural anisotropy and therefore optical appearance of the colloidal dispersion in a magnetic field. We also found that Ti(IV/III)Nss is autoxidized back to Ti(IV)NSs under non-deaerated conditions. By using an elaborate setup, the dispersion of Ti(IV)NSs serves as an optical switch remotely operable by magnet and light.